This invention relates generally to high intensity discharge lamps wherein the arc discharge is generated by a plasma in a solenoidal electric field and more particularly to use of a new buffer gas employed in the arc tube fill in combination with sodium iodide or the combination of sodium iodide and cerium halide to improve starting performance without adversely affecting lamp efficacy or color rendition. Lamp efficiency or "efficacy", as used in the present application, means luminous efficacy as measured in conventional terms of lumens per watt. As to color rendition, general purpose illumination requires that objects illuminated by a particular light source display much the same color as when illuminated by natural sunlight. Such requirement is measured by known standards such as the C.I.E. color rendering index values (CRI), and CRI values of 50 or greater are deemed essential for commercial acceptability of lamps in most general lighting applications. A still further requirement for commercially acceptable general purpose illumination is the white color temperature provided with such lamp, which is fixed at about 3000.degree.K for the warm white lamp, about 3500.degree.K for the standard white lamp and about 4200.degree.K for the cool white lamp, as measured by the C.I.E. chromaticity x and y values.
The lamps described in the present invention are part of the class referred to as high intensity discharge lamps (HID) because in their basic operation a medium to high pressure gas is caused to emit visible wavelength radiation upon excitation typically caused by passage of current through an ionizable gas such as sodium vapor or mixed sodium vapor and cerium vapor. The original class of such HID lamps was that in which the discharge current was caused to flow between a pair of electrodes. Since the electrode members in such electroded HID lamps were prone to vigorous attack by the arc tube fill materials, causing early lamp failure, the more recently developed solenoidal electric field lamps of this type have been proposed to broaden the choice of arc tube materials through elimination of the electrode component. Such more recently developed solenoidal electric field lamps are described in J.M. Anderson U.S. Pat. Nos. 4,017,764 and 4,180,763, and Chalek and Johnson U.S. Pat. No. 4,591,759, all assigned to the assignee of the present invention, and generate a plasma arc in the arc tube component during lamp operation, all in a previously known manner.
Conventional electrodeless HID lamps suffer from the disadvantage that they are difficult to start. This is because the xenon buffer gas also functions as the starting gas. However, xenon is difficult to start, especially when used at a high pressure, such as 200 torr, as compared with the more conventional starting gas pressures of 30 torr or less. The difficulty of starting high-pressure xenon, combined with the low solenoidal electric field available from the lamp induction coil, has heretofore made room temperature HID lamp starting impossible.
One method that has been used for starting HID lamps involves immersing the arc tube in liquid nitrogen so as to condense most of the xenon. Thereafter, the induction coil current is increased, and the lamp usually starts at a current of 18 amps or less. If necessary, a spark coil is used to apply high-voltage pulses to help start the discharge. Once the lamp is started, heat from the discharge evaporates the condensed xenon and normal xenon pressure is reached.
The liquid nitrogen method is effective because there is an optimum xenon pressure for starting the discharge. While this optimum pressure is not known with great precision for the above stated starting conditions, it is nevertheless well below 200 torr and above the saturation vapor pressure of xenon (2.5 millitorr) at the temperature of liquid nitrogen (77.degree.K). Since the liquid nitrogen starting method is clearly not practical for commercial lamps, it is desirable to employ a more practical starting method for room-temperature operated HID lamps.